Near-field photomask, near-field exposure apparatus using the photomask, dot pattern forming method using the exposure apparatus, and device manufactured using the method

ABSTRACT

A near-field photomask is made up of a light shield film and openings formed in the light shield film, the photomask being used to expose an exposure target with a near-field light generated through the openings. The openings formed in the light shield film comprise two or more parallel rows of first slit openings each having a width smaller than 100 nm, and a second slit opening having a width smaller than 100 nm and extended perpendicularly to the rows of first slit openings while interlinking at least two of the rows of first slit openings.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exposure apparatus forforming a micropattern, such as required in a quantum dot device and asub-wavelength structured device, and also relates to a mask for use inthe exposure apparatus.

[0003] 2. Description of the Related Art

[0004] With an increasing capacity of a semiconductor memory anddevelopment toward higher speeds and higher packing density of CPUprocessors, it is essential to realize a finer microstructure withphotolithography. A limit in micromachining achievable with aphotolithography apparatus is generally on the order of about thewavelength of light used.

[0005] For that reason, the wavelength of light used in thephotolithography apparatus has been shortened. At the present, a nearultraviolet laser is employed as a light source and micromachining onthe order of 0.1 μpm is realized.

[0006] Thus, photolithography capable of providing a finermicrostructure has been developed. To carry out the micromachining onthe order of not larger than 0.1 μm, however, problems to be solvedarise in points of, e.g., using a laser of shorter wavelength anddeveloping a lens in such a wavelength range.

[0007] Meanwhile, a micromachining apparatus is proposed in which ascanning near-field optical microscope (abbreviated to “SNOM”hereinafter) is employed as means for enabling the micromachining on theorder of not larger than 0.1 μm to be performed with the use of light.In the proposed apparatus, local exposure beyond a limit imposed by thewavelength of light is carried out on a resist by using a near-fieldlight protruded from a micro-opening with a size of not larger than 100nm, for example.

[0008] In any of photolithography apparatuses using the SNOM, however,the micromachining is performed in a similar manner to drawing with asingle stroke by employing one (or several) optical probe. Hence, aproblem arises in that a throughput is not so improved.

[0009] As one method for overcoming such a problem, a near-field maskexposure technique is proposed in which a light is illuminated to a rearsurface of a photomask prepared by forming a pattern of openings of notlarger than 0.1 μm in a light shield film, and transferring a pattern ofth photomask onto a resist at a time by using a near-field lightprotruded through the opening pattern (see U.S. Pat. No. 6,171,730). Theinvention disclosed in U.S. Pat. No. 6,171,730 is very superior andgives a great contribution to the fields of photolithography andsemiconductor production technology.

[0010] On the other hand, it is also proposed to employ a mask pattern,which has dot openings 1001 arranged as shown in FIG. 10, when forming adot pattern with exposure based on the near-field mask exposurementioned above.

[0011] However, if the opening size is reduced to form a dot pattern ofa smaller size, there arises a risk that the intensity of a near-fieldlight generated from the opening is reduced and a time required for theexposure is prolonged, thus resulting in a reduced throughput of themicromachining apparatus.

SUMMARY OF THE INVENTION

[0012] To overcome the above-mentioned problems in the art, according toa first aspect, the present invention provides a near-field photomaskmade up of a light shield film and openings formed in the light shieldfilm, the photomask being used to expose an exposure target with anear-field light generated through the openings, wherein the openingsformed in the light shield film comprise two or more parallel rows offirst slit openings each having a width smaller than 100 nm, and asecond slit opening having a width smaller than 100 nm and extendedperpendicularly to the rows of first slit openings while interlinking atleast two of the rows of first slit openings.

[0013] According to a second aspect, the present invention provides anear-field exposure apparatus comprising a near-field photomaskaccording to the first aspect; a light illuminating unit forilluminating a polarized light, which has an electric field componentparallel to the rows of first slit openings, to the near-fieldphotomask; and a unit for positioning the near-field photomask close tothe exposure target up to a distance within a near-field region.

[0014] According to a third aspect, the present invention provides a dotpattern forming method including a step of forming a dot pattern byusing the near-field exposure apparatus according to the second aspect.According to a fourth aspect, the present invention provides a devicemanufactured by using the near-field exposure apparatus according to thesecond aspect.

[0015] Thus, with the near-field photomask of the present invention, anopening pattern formed in the light shield film comprise two or morerows of first slit openings each having a width smaller than 100 nm andbeing parallel to the direction of an electric field of an incidentlight, and a second slit opening having a width smaller than 100 nm andextended perpendicularly to the direction of the electric field of theincident light while interlinking at least two of the rows of first slitopenings. It is therefore possible to increase the intensity of anear-field light generated through the opening pattern, and to improve athroughput in the process using exposure. In addition, the cost of adevice manufactured using the near-field photomask can be reduced.

[0016] Further objects, features and advantages of the present inventionwill become apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an illustration for explaining the principle of anear-field photomask according to the present invention.

[0018]FIG. 2 is an illustration showing a first embodiment of thenear-field photomask according to the present invention.

[0019]FIG. 3 is an illustration showing a second embodiment of thenear-field photomask according to the present invention.

[0020]FIG. 4 is an illustration showing a third embodiment of thenear-field photomask according to the present invention.

[0021]FIG. 5 is an illustration showing a fourth embodiment of thenear-field photomask according to the present invention.

[0022]FIGS. 6A and 6B are a plan view and a sectional view showing aconstruction of the near-field photomask.

[0023]FIG. 7 is a schematic view showing a construction of a near-fieldphotomask exposure apparatus.

[0024]FIG. 8 is an illustration for explaining the principle ofnear-field exposure.

[0025]FIGS. 9A to 9E are schematic views for explaining a method ofmanufacturing the near-field photomask.

[0026]FIG. 10 is an illustration for explaining a near-field photomaskof prior art.

[0027]FIGS. 11A and 11B are a sectional view and a plan view of anactive layer portion showing a construction of a quantum dotsuper-lattice device manufactured using the exposure apparatus andmethod of the present invention.

[0028]FIGS. 12A and 12B are a perspective view and a sectional viewshowing a construction of a sub-wavelength optical device manufacturedusing the exposure apparatus and method of the present invention.

[0029]FIGS. 13A and 13B are a perspective view and a sectional viewshowing a construction of a sensor manufactured using the exposureapparatus and method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] As mentioned in the summary, the first aspect of the presentinvention resides in a near-field photomask made up of a light shieldfilm and openings formed in the light shield film, the photomask beingused to expose an exposure target with a near-field light generatedthrough the openings, wherein the openings formed in the light shieldfilm comprise two or more parallel rows of first slit openings eachhaving a width smaller than 100 nm, and a second slit opening having awidth smaller than 100 nm and extended perpendicularly to the rows offirst slit openings while interlinking at least two of the rows of firstslit openings.

[0031] The second aspect of the present invention resides in anear-field exposure apparatus comprising a near-field photomaskaccording to the first aspect; a light illuminating unit forilluminating a polarized light, which has an electric field componentparallel to the rows of first slit openings, to the near-fieldphotomask; and a unit for positioning the near-field photomask close tothe exposure target up to a distance within a near-field region.

[0032] The third aspect of the present invention resides in a dotpattern forming method including a step of forming a dot pattern byusing the near-field exposure apparatus according to the second aspect.The fourth aspect of the present invention resides in a devicemanufactured by using the near-field exposure apparatus according to thesecond aspect.

[0033] The present invention will be described below in more detail.

[0034] (Mask)

[0035] When a light of wavelength ranging from a visible to nearultraviolet range is illuminated to a slit opening with a width of notlarger than 100 nm, the intensity of generated near-field light is smallif the direction of an electric field of the incident light is parallelto the slit opening, and it is large if the direction of an electricfield of the incident light is perpendicular to the slit opening. Thisresult is attributable to that the near-field light is more intensifiedat edges of the slit opening parallel to the direction of the electricfield of the incident light than at edges of the slit openingperpendicular to the direction of the electric field of the incidentlight.

[0036] Therefore, a latent image of a pattern of only the slit openingsperpendicular to the direction of the electric field of the incidentlight can be formed in a photoresist used in the near-field exposure byadjusting the intensity of exposure light such that the intensity of thenear-field light generated when the direction of the electric field ofthe incident light is parallel to the slit openings is smaller than athreshold for the formation of the latent image, and the intensity ofthe near-field light generated when the direction of the electric fieldof the incident light is perpendicular to the slit openings is largerthan the threshold for the formation of the latent image.

[0037] As shown in FIG. 1, for example, rows of first slit openings 103are formed in a light shield film 102 of a is near-field photomask 101parallel to the direction of the electric field of the incident light,and second slit openings 104 are formed in the light shield film 102perpendicularly to the direction of the electric field of the incidentlight so as to interlink plural rows of the first slit openings 103.

[0038] For the sake of explanation, the second slit openings 104 areeach shown in FIG. 1 as being demarcated by dotted lines and having anarea slightly smaller than the actually defined opening. The second slitopening 104 may interlink two rows of the first slit openings 103 or,three or more rows of the first slit openings 103.

[0039] Numeral 105 in FIG. 1 shows a latent image pattern formed in thephotoresist as a result of subjecting the photoresist to exposurethrough the near-field photomask 101. As shown, a latent image is formedin each area of the near-field photomask 101 where a light shieldportion 106 and the second slit opening 104 cross each other. Numeral107 denotes the area where the latent image is formed. Such formation ofthe latent image is attributable to that, as described above, thenear-field light is more intensified at edges of the slit openingparallel to the direction of the electric field of the incident lightthan at edges of the slit opening perpendicular to the direction of theelectric field of the incident light, and that there are no edges of theslit opening in the area where the row of first slit opening and thesecond slit opening cross each other.

[0040] The intensity of the near-field light generated in the area ofthe near-field photomask 101 where the light shield portion 106 and thesecond slit opening 104 cross each other, as described above, is largerthan the intensity of near-field light generated from a single dotopening of the same size as the crossed area. The reason is that nolight shield films exist in the vicinity around the crossed area in adirection perpendicular to the direction of the electric field of theincident light and hence the shield effect developed by movements ofelectrons in the light shield film does not act upon the crossed area.

[0041]FIGS. 6A and 6B show details of one example of construction of thenear-field photomask of the present invention. FIG. 6A is a plan view ofthe photomask, looking at the mask front side, FIG. 6B is a sectionalview of the near-field photomask.

[0042] As sown in FIGS. 6A and 6B, the near-field photomask is made upof a mask base 601 formed of a thin film with a thickness from 0.1 to 1μm and being transparent to a light of exposure wavelength, and a metalthin film 602 having a thickness from about 5 to 100 nm and formed onthe mask base 601. A micro-opening pattern 603 with a width of notlarger than 100 nm is formed in the metal thin film 602. The mask base601 is supported on a substrate 604.

[0043] If the mask base 601 has a smaller thickness, it is more easilysusceptible to an elastic deformation and is able to elastically deformmore closely following smaller irregularities and undulations in boththe surfaces of the resist and the substrate, thereby improvingcloseness. However, if the thickness of the mask base 601 is too smallwith respect to the exposure area, a trouble may occur in that thestrength required for the mask base 601 to serve as a mask becomesdeficient, or that the mask base 601 is broken by attraction forcesacting between the resist and the substrate when peeling off the maskbase 601 after the steps of closely positioning and exposing the resist.

[0044] From the viewpoint of the above-described mechanicalcharacteristics, the thickness of the mask base 601 is preferably in therange of 0.1 to 100 μm.

[0045] (Exposure Apparatus)

[0046] One example of an exposure apparatus using the near-fieldphotomask of the present invention will be described below.

[0047]FIG. 7 shows a construction of a near-field photomask exposureapparatus using the near-field photomask of the present invention.Referring to FIG. 7, numeral 701 denotes a near-field photomask that isemployed as a mask for exposure in the present invention. The near-fieldphotomask 701 is made up of a mask base 702 and a metal thin film 703.The metal thin film 703 is formed on the mask base 702, and amicro-opening pattern 704 is formed in the metal thin film 703.

[0048] The near-field photomask 701 is disposed such that the front side(lower side in FIG. 7) is positioned outside a pressure adjusting vessel705 and the rear side (upper side in FIG. 7) is positioned to face aninner space of the pressure adjusting vessel 705. The pressure adjustingvessel 705 is constructed to be able to adjust a pressure in the innerspace.

[0049] Numeral 709 denotes an exposure light source. A light emittedfrom the light source turns to an exposure light 710 through a polarizer714 and a collimator lens 711. The exposure light 710 is illuminated tothe photomask 701 through a glass window 712 of the pressure adjustingvessel 705. The polarizer 714 has an axis of polarization set such thatthe direction of the electric field of the light having passed thepolarizer 714 is parallel to rows of first slit openings formed in thephotomask 701.

[0050] A target subjected to the exposure is prepared by forming aresist 707 on the surface of a substrate 706. A composite of the resist707 and the substrate 706 is mounted on a stage 708. The stage 708 isdriven to move for relative positioning between the substrate 706 andthe near-field photomask 701 in two-dimensional directions within a maskplane. Then, the stage 708 is driven to move in the direction normal tothe mask plane, causing the near-field photomask 701 and the resist 707on the substrate 706 to position close to such an extent that a gapbetween the front surface of the near-field photomask 701 and thesurface of the resist 707 is not larger than 100 nm all over theinterface.

[0051] Subsequently, the exposure light 710 emitted from the exposurelight source 709 is introduced to the pressure adjusting vessel 705through the glass window 712 and is illuminated to the near-fieldexposure photomask 701 from the rear side (upper side in FIG. 7). Theresist 707 is thereby exposed to a near-field light protruded throughthe micro-opening pattern 704 formed in the metal thin film 703 that isformed on the mask base 702 on the front side of the near-field exposurephotomask 701.

[0052] The resist 707 can be made of a material selected from amongphotoresist materials used in ordinary semiconductor processes. Thewavelength of light capable of exposing the photoresist materials isapproximately in the range of 200 to 500 nm. Selecting those of thephotoresist materials, which are sensitive to the g-line and the i-lineparticularly in the range of 350 to 450 nm, is advantageous in that,because those photoresist materials are available in various types atrelatively low costs, the flexibility in process design is increased andthe production cost can be reduced.

[0053] The resist 707 made of one of those photoresist materials hasspecific exposure sensitivity Eth (=threshold for the exposure)depending on the film thickness and the wavelength of the exposurelight. The resist 707 is exposed by illuminating the light with theintensity of not lower than the exposure sensitivity Eth. In thenear-field optical exposure apparatus of the present invention, asdescribed later in detail, the resist 707 preferably has a filmthickness of not larger than 100 nm. When the above-mentionedphotoresist materials are used at a film thickness of not larger than100 nm, the exposure sensitivity Eth is approximately in the range of 5to 50 mJ/cm².

[0054] The exposure light source 709 is required to illuminate a lightin a wavelength range capable of exposing the resist 707 used. Forexample, when one of the above-mentioned photoresist materials sensitiveto the g-line and the i-line is selected to form the resist 707, theexposure light source 709 may be constituted by one of a HeCd laser(light wavelength: 325 nm and 442 nm), a GaN-based blue semiconductorlaser (light wavelength: shorter than 410 nm), second harmonic (SHG) andthird harmonic (THG) lasers among infrared lasers, and a mercury lamp(g-line: 436 nm and i-line: 365 nm).

[0055] The intensity of exposure light is regulated by adjusting thedriving voltage, the driving current and the illumination time of theexposure light source 709. For example, when a beam of the HeCd laserwith a beam diameter of 1 mm and an optical output of 100 mW is enlargedso as to cover an area of 100 mm×100 mm by using a beam expander and acollimator lens, optical power per unit area is 1 mW/cm². When thethus-obtained light is illuminated to the resist surface for 10 seconds,the intensity of light exposed to the resist is 10 mJ/cm². If theintensity of the exposure light exceeds the above-mentioned exposuresensitivity Eth of the resist 707, the exposure of the resist isperformed.

[0056] In practice, because the light is illuminated to the resist 707through the near-field exposure photomask 701, the optical power must beadjusted taking into account the transmittance of the photomask 701.

[0057] (Principle of Near-field Exposure)

[0058] The principle of exposure using the near-field light is nowdescribed with reference to FIG. 8.

[0059] Referring to FIG. 8, an incident light 803 having entered a maskbase 802, which constitutes a photomask 801 for use in the near-fieldexposure, illuminates a micro-opening pattern 805 formed in a metal thinfilm 804. The opening size (width) of the micro-opening pattern 805 issmaller than the wavelength of the incident light 803 and is not largerthan 100 nm.

[0060] Usually, a light hardly passes through an opening with a sizesmaller than the wavelength of the light, but a slight amount of light,called a near-field light 806, is protruded through the opening in thevicinity thereof. The near-field light 806 is a non-propagating lightthat is generated only in the vicinity of the opening within a distanceof not larger than about 100 nm from the opening, and has a propertythat the intensity of the near-field light is abruptly reduced as thedistance from the opening increases.

[0061] Then, the surface of a resist 808 substrate 807 is positionedcloser to the micro-opening pattern 805, through which the near-fieldlight 806 is protruded, within a distance of not larger than about 100nm from the pattern 805. As a result, the near-field light 806 isscattered in the resist 808 and is converted into a propagating lightwhich exposes the resist 808.

[0062] Assuming here that the thickness of the metal thin film 804 isabout 100 nm, the metal thin film 804 is able to shield almost 100% of atransmitted light that is resulted from the incident light 803 directlypassing through an area of the metal thin film 804 other than an area inwhich the micro-opening pattern 805 is formed. Thus, the metal thin film804 is able to prevent the surface of the resist 808 from being exposedin its area other than an area positioned to face the area of the metalthin film 804 in which the micro-opening pattern 805 is formed.

[0063] To form a pattern having a smaller line width, however, theopening width of the micro-opening pattern 805 must be reduced, i.e.,the aspect ratio of the micro-opening pattern 805 (=thickness of themetal thin film 804/the opening width of the micro-opening pattern 805)must be increased.

[0064] In the above-described exposure method using the near-field light806 protruded through the micro-opening pattern 805, as the openingwidth of the micro-opening pattern 805 decreases, the intensity of thenear-field light 806 is reduced. Also, assuming the opening width to bethe same, as the thickness of the metal thin film 804 increases, theintensity of the near-field light 806 is reduced. This result isattributable to that the longer a passage having a width smaller thanthe wavelength of light, the harder is the near-field light 806 toprotrude through the micro-opening pattern 805. From those reasons, arequired exposure time is increased as the opening width of themicro-opening pattern 805 reduces.

[0065] Further, reducing the opening width of the micro-opening pattern805 increases the aspect ratio and hence requires a highly advancedtechnique for manufacturing the near-field exposure photomask 801, thusresulting a reduction of yield.

[0066] If the film thickness of the resist 808 is sufficiently small,scattering of the near-field light 806 in the resist 808 is preventedfrom spreading in the planar (horizontal) direction to a large extent,and a latent image of a micro-pattern defined by the micro-openingpattern 805 with the opening size smaller than the wavelength of theincident light 803 can be transferred to and formed in the resist 808.

[0067] After exposing the resist 808 with the near-field light 806 asdescribed above, the substrate 807 is processed by using ordinarysemiconductor processes. For example, a micro-pattern defined by themicro-opening pattern 805 is formed on the substrate 807 through stepsof developing the resist and carrying out etching.

[0068] (Closely Positioning Method)

[0069] A method of positioning the near-field photomask and theresist-and-substrate composite close to each other will be described indetail below with reference to FIG. 7.

[0070] If the front surface of the photomask 701 for use in thenear-field exposure and the surface of the resist 707 are each perfectlyflat, both the surfaces can be positioned close to each other over theentire interface.

[0071] In practice, however, the photomask surface and the surface ofthe resist-and-substrate composite inevitably include irregularities andundulations. Accordingly, if the photomask and the resist are merelybrought into a closely positioned or a contact state, there is a riskthat a contact area and a non-contact area coexist between them.

[0072] To avoid such a risk, a pressure is applied in the direction fromthe rear side to the front side of the near-field exposure photomask701, thereby causing the near-field exposure photomask 701 to flex withan elastic deformation. The photomask 701 is thus pressed against thecomposite of the resist 707 and the substrate 706 so that they can bepositioned close to each other over the entire surfaces.

[0073] As one example of the method of applying a pressure, thenear-field exposure photomask 701 is disposed, as shown in FIG. 7, suchthat the front side of the photomask 701 is positioned outside thepressure adjusting vessel 705 and the rear side thereof is positioned toface the inner space of the pressure adjusting vessel 705. Then, a highpressure gas is introduced to the inner space of the pressure adjustingvessel 705 by using a pressure adjusting unit 713, e.g., a pump, so asto create a higher pressure in the pressure adjusting vessel 705 thanthe atmospheric pressure.

[0074] As another example, the inner space of the pressure adjustingvessel 705 may be filled with a liquid transparent to the exposure light710, and a pressure of the liquid within the pressure adjusting vessel705 may be adjusted by using a cylinder.

[0075] By introducing the high pressure gas to the inner space of thepressure adjusting vessel 705 with the pressure adjusting unit 713 andincreasing the pressure in the pressure adjusting vessel 705, the frontsurface of the near-field photomask 701 and the surface of the resist707 on the substrate 706 can be positioned close to each other over theentirety thereof under a uniform pressure.

[0076] By applying a pressure in such a manner, repulsive forces actingbetween the front surface of the near-field photomask 701 and thesurface of the resist 707 on the substrate 706 become uniform on thebasis of the Pascal's principle. This results in advantages that largeforces are avoided from being locally applied to both the surfaces ofthe near-field photomask 701 and the resist 707 on the substrate 706,and hence that the near-field photomask 701, the substrate 706 and theresist 707 are prevented from being locally broken.

[0077] In this respect, by adjusting the pressure in the pressureadjusting vessel 705, the pressing forces acting between the near-fieldphotomask 701 and the composite of the resist 707 and the substrate 706,i.e., contact forces between them, can be controlled. For example, inthe case in which the surfaces of the photomask, the resist and thesubstrate include relatively large sizes of irregularities andundulations, by setting the pressure in the pressure adjusting vessel705 to a higher value, it is possible to increase the contact forces andto eliminate variations in the gap between both the surfaces of thenear-field photomask and the resist-and-substrate composite, which arecaused by irregularities and undulations in the respective surfaces.

[0078] In this embodiment, the description is made of the example inwhich the rear side of the near-field photomask is arranged to positioninside the pressurized vessel, thus applying a pressure from the rearside to the front side of the near-field photomask based on a pressuredifference produced between the pressure in the pressurized vessel andthe atmospheric pressure at a relatively lower level so that thenear-field photomask and the composite of the resist and the substrateare positioned close to each other. Conversely, the front side of thenear-field photomask and the resist-and-substrate composite may bearranged to position inside a depressurized vessel, thus applying apressure from the rear side to the front side of the near-fieldphotomask based on a pressure difference produced between the pressurein the depressurized vessel and the atmospheric pressure at a relativelyhigher level.

[0079] (Peeling-Off Method)

[0080] Peeling-off of the near-field photomask from the composite of theresist and the substrate after the completion of the near-field exposureis carried out as follows.

[0081] The pressure in the pressure adjusting vessel 705 is reduced to alevel lower than the atmospheric pressure by using the pressureadjusting unit 713. A resulting pressure difference causes the surfaceof the metal thin film 703 on the near-field photomask 701 to peel offfrom the surface of the resist 707 on the substrate 706.

[0082] When peeling off the near-field photomask 701 from the compositeof the resist 707 and the substrate 706 by reducing the pressure asdescribed above, attraction forces acting between the front surface ofthe near-field photomask 701 and the surface of the resist 707 on thesubstrate 706 become uniform on the basis of the Pascal's principle.This results in advantages that large forces are avoided from beinglocally applied to both the surfaces of the near-field photomask 701 andthe resist 707 on the substrate 706, and hence that the near-fieldphotomask 701, the substrate 706 and the resist 707 are prevented frombeing locally broken.

[0083] In this respect, by adjusting the pressure in the pressureadjusting vessel 705, the forces attracting the near-field photomask 701away from the composite of the resist 707 and the substrate 706, i.e.,the tensile forces for separating them, can be controlled. For example,in the case in which the contact forces between both the surfaces of thephotomask and the resist-and-substrate composite are large, by settingthe pressure in the pressure adjusting vessel 705 to a lower level, itis possible to increase the tensile forces and to more easily peel offthe near-field photomask from the resist-and-substrate composite.

[0084] As mentioned above, the apparatus for applying a pressure in thestep of positioning the near-field photomask and theresist-and-substrate composite close to each other may be constructed,conversely to the construction shown in FIG. 7, such that the front sideof the near-field photomask and the resist-and-substrate composite maybe both arranged to position inside the depressurized vessel, thusapplying a pressure from the rear side to the front side of thenear-field photomask based on a pressure difference between the pressurein the depressurized vessel and the atmospheric pressure at a relativelyhigher level. In that case, the pressure in the depressurized vessel isset to a level higher than the atmospheric pressure in the peeling-offstep.

[0085] An any case, the peeling-off step can be performed by producing apressure difference such that the rear side of the near-field photomaskis subjected to a lower pressure than the front side thereof.

[0086] (Mask Manufacturing Method)

[0087] A method of manufacturing the near-field photomask of the presentinvention will be described in detail below with reference to FIGS. 9Ato 9E.

[0088] As shown in FIG. 9A, a Si₃N₄ film 902 serving as a mask base anda Si₃N₄ film 903 for forming an etching window therein are each formedby the LP-CVD method at a film thickness of 2 μm respectively on a frontsurface (upper surface in FIG. 9) and a rear surface (lower surface inFIG. 9) of a Si(100)-substrate 901 with a thickness of 500 μm, both thefront and rear surfaces being polished. Then, a Cr thin film 904 isformed, as a metal thin film for forming a micro-opening patterntherein, by the vapor deposition method at a film thickness of 50 nmover the Si₃N₄ film 902 on the front surface under control with a filmthickness monitor using a quartz oscillator.

[0089] Then, a resist 905 sensitive to an electron ray is coated on thefront surface, and a drawing pattern 907 is formed at a width of 10 nmby using an electron ray beam 906 (FIG. 9B). After developing the resist905, the substrate is subjected to dry etching to form a micro-openingpattern 907 in the Cr thin film 904 (FIG. 9C).

[0090] Subsequently, an etching window 909 is formed in the Si₃N₄ film903 on the rear side (FIG. 9C). Anisotropic etching is performed on therear side of the Si substrate 901 by using KOH, thereby obtaining a mask910 in the form of a thin film (FIG. 9D).

[0091] Finally, the mask 910 is bonded to a mask support member 911 toobtain a near-field photomask (FIG. 9E).

[0092] In this embodiment, the description is made of the example inwhich the micromachining method with an electron ray is employed in thestep of forming the micro-opening pattern 908 in the Cr thin film 904.However, the converged ion beam machining method, the X-ray lithography,or the scanned probe microscope (SPM) machining method may be usedinstead of the electron ray machining method. Above all, by forming themicro-opening pattern with the machining method based on the SPMtechnology represented by a scanned tunnel microscope (STM), an atomicforce microscope (AFM), and a scanning near-field optical microscope(SNOM), a submicro-opening pattern with an opening size of not largerthan 10 nm can be formed. Thus, the SPM technology is also amicromachining method very suitable in implementing the presentinvention.

[0093] (Exposure Method)

[0094] This embodiment has been described in connection with the examplein which the mask base is formed as a thin film and is elasticallydeformed so that the entire surface of the photomask is positioned closeto the resist surface following a surface shape of the resist.

[0095] The concept of the present invention is not limited to theabove-mentioned embodiment, but it is also applicable to the case ofusing a mask base which is not in the form of a thin film and hasrigidity. This case however accompanies a risk that the closenessbetween the photomask surface and the resist surface over the entireinterface may deteriorate from a satisfactory level.

[0096] Also, if the surface of the metal thin film 602 on the sidepositioned close to the resist-and-substrate composite is not flat,there is a risk that the photomask is not positioned satisfactorilyclose to the resist-and-substrate composite, and hence a variation inexposure may occur.

[0097] For that reason, it is desired that the surface of the metal thinfilm 602 be highly flat with irregularities reduced to the order of notlarger than at least 100 nm, preferably not larger than 10 nm.

[0098] The opening width of the micro-opening pattern is set to besmaller than the wavelength of the light for use in the exposure andequal to a desired pattern width of the exposure performed on theresist. More specifically, the opening width of the micro-openingpattern is preferably selected from the range of 1 to 100 nm. If theopening width of the micro-opening pattern is not smaller than 100 nm,an undesired result would occur in that not only the near-field lightintended in the present invention, but also the direct propagating lightof higher intensity pass through the photomask. On the other hand, ifthe opening width of the micro-opening pattern is not larger than 1 nm,it would be not practical, though the exposure is feasible, because theintensity of the near-field light protruded through the photomask isvery small and a longer time is required for the exposure.

[0099] While the opening width of the micro-opening pattern requires tobe not larger than 100 nm as described above, the length of the openingin the lengthwise direction has no limitations and a free pattern shapecan be selected. The micro-opening pattern can be a key-shaped pattern,by way of example, as shown in FIG. 6A. Alternatively, it may be anS-shaped pattern (though not shown).

[0100] The substrate 206 employed as a workpiece in the near-fieldexposure apparatus of the present invention can be selected from among avariety of substrates, such as semiconductor substrates made of Si,GaAs, InP, etc., insulating substrates made of glass, quartz, BN, etc.,and substrates obtained by forming films of metals, oxides, nitrides,etc. on those substrates.

[0101] In the near-field exposure apparatus of the present invention, itis desired that the photomask 701 for use in the near-field exposure andthe composite of the resist 707 and the substrate 706 be positionedclose to each other to such an extent that the gap between them is heldnot larger than at least 100 nm, preferably not larger than 10 nm, overthe entire exposure region. The substrate selected for practical use isdesirably as possible as flat.

[0102] Similarly, it is desired that the surface of the resist 707 usedin the present invention be flat and have very small irregularities.Also, because the light protruded through the near-field photomask 701attenuates exponentially as the distance from the photomask increases,the near-field light is hard to expose the resist 707 up to a depth ofnot smaller than 100 nm and tends to spread in the resist whilescattering, thereby increasing the exposure pattern width. Inconsideration of those properties of the near-field light, it is desiredthat the thickness of the resist 707 be not larger than at least 100 nm,preferably as thin as possible.

[0103] Thus, the resist material coating method is advantageouslyselected to be capable of forming the resist such that the filmthickness of the resist is not larger than at least 100 nm, preferablynot larger than 10 nm, and that the resist surface is very flat and hasirregularities with sizes of not larger than at least 100 nm, preferablynot larger than 10 nm.

[0104] As an alternative method of satisfying those conditions, theresist material may be dissolved in a solvent so as to have lowerviscosity, and then spin-coated on the substrate at a very small anduniform thickness.

[0105] As still another resist material coating method, the LB (LangmuirBlodgett) method may also be used in which an accumulated film made upof monolayers is formed on a substrate by scooping, onto the substrate,a monolayer at the predetermined number of times, which is obtained byarranging amphiphilic resist material molecules on the water surface,each of the molecules containing a hydrophobic group and a hydrophilicfunction group.

[0106] Furthermore, the SAM (Self Assemble Monolayer) forming method maybe used instead in which a monolayer of a potoresist material is formedon a substrate by depositing only a single molecule layer with physicaladsorption or chemical coupling on the substrate in a solution or a gasphase.

[0107] Of the coating methods described above, the LB method and the SAMmethod are able to form a very thin resist film at a uniform thicknessand high surface flatness. They are therefore resist material coatingmethods highly suitable for use with the near-field exposure apparatusof the present invention.

[0108] In the near-field exposure, it is desired that the gap betweenthe near-field photomask 701 and the composite of the resist 707 and thesubstrate 706 be not larger than 100 nm and be held uniform withoutvariations over the entire exposure region.

[0109] It is therefore not preferable that a pattern havingirregularities already formed in another lithographic process is presenton a substrate used in the near-field exposure and the substrate surfacehas irregularities with sizes of not smaller than 100 nm.

[0110] In other words, the substrate used in the near-field exposure ispreferably one which has not been subjected to other various processes,i.e., it is in an initial process stage and is as flat as possible.Accordingly, when the near-field exposure process is combined with otherlithography processes, the near-field exposure process is desirablycarried out in a stage as early as possible.

[0111] Further, in FIG. 8, the intensity of the near-field light 806protruded through the micro-opening pattern 805 on the near-fieldphotomask 801 differs depending on the opening size of the micro-openingpattern 805. Therefore, if the micro-openings have different sizes fromeach other, the intensity of exposure performed on the resist 808 alsovaries and a uniform pattern is difficult to form.

[0112] To avoid such variations in opening size, it is desired that themicro-opening pattern formed in the near-field photomask used in eachstep of near-field exposure process have the uniform opening width.

[0113] The above description has been made of the apparatus in which thenear-field photomask covering the entire substrate surface is employedand the near-field exposure is performed over the entire substratesurface at a time. The concept of the present invention is not limitedto that type of apparatus, but it is also applicable to astep-and-repeat apparatus in which the near-field photomask with a sizesmaller than the substrate is employed and the near-field exposure for apart of the substrate is repeated while changing the exposure positionon the substrate.

EMBODIMENTS

[0114] (First Embodiment)

[0115]FIG. 2 shows a first embodiment of the opening pattern for thenear-field photomask of the present invention.

[0116] Referring to FIG. 2, areas 208 where light shield portions 206and second slit openings 204 in a near-field photomask 201 cross eachother are arrayed in a rectangular lattice pattern. By exposing aphotoresist with the use of the near-field photomask 201, a latent imagepattern is formed as a pattern of dots arrayed in the form of arectangular lattice as denoted by numeral 205.

[0117] The dimensions of the opening pattern formed in the near-fieldphotomask 201 are, by way of example, as follows. A row of first slitopening 203 has a width of 40 nm, and the light shield portion 206 has awidth of 20 nm. The second slit opening 204 has a width of 20 nm, theinterval between adjacent two of a plurality of second slit openings 204is 60 nm, and the crossed area 208 has a size of 20 nm×20 nm. In thiscase, the latent image pattern 205 formed in the photoresist is made upof a plurality of latent-dot-image formed areas 207 each having a sizeof about 30 nm×30 nm, which are arranged in a two-dimensional array witha pitch of 80 nm in the vertical direction and a pitch of 60 nm in thehorizontal direction as viewed in FIG. 2.

[0118] (Second Embodiment)

[0119]FIG. 3 shows a second embodiment of the opening pattern for thenear-field photomask of the present invention.

[0120] Referring to FIG. 3, areas 308 where light shield portions 306and second slit openings 304 in a near-field photomask 301 cross eachother are arrayed in a triangular lattice pattern. By exposing aphotoresist with the use of the near-field photomask 301, a latent imagepattern is formed in the photoresist as a pattern of dots arrayed in theform of a triangular lattice as denoted by numeral 305.

[0121] The dimensions of the opening pattern formed in the near-fieldphotomask 301 are, by way of example, as follows. A row of first slitopening 303 has a width of 40 nm, and the light shield portion 306 has awidth of 20 nm. The second slit opening 304 has a width of 20 nm, theinterval between adjacent two of a plurality of second slit openings 304is 60 nm, and the crossed area 308 has a size of 20 nm×20 nm. In thiscase, the latent image pattern 305 formed in the photoresist is made upof a plurality of latent-dot-image formed areas 307 each having a sizeof about 30 nm×30 nm, which are arranged in a triangular lattice arraywith a pitch of 100 nm in the oblique direction and a pitch of 120 nm inthe horizontal direction as viewed in FIG. 3.

[0122] (Third Embodiment)

[0123]FIG. 4 shows a third embodiment of the opening pattern for thenear-field photomask of the present invention.

[0124] Referring to FIG. 4, areas 408 where light shield portions 406and second slit openings 404 in a near-field photomask 401 cross eachother are arrayed in a hexagonal lattice pattern. By exposing aphotoresist with the use of the near-field photomask 401, a latent imagepattern is formed in the photoresist as a pattern of dots arrayed in theform of a hexagonal lattice as denoted by numeral 405.

[0125] The dimensions of the opening pattern formed in the near-fieldphotomask 401 are, by way of example, as follows. A row of first slitopening 403 has a width of 40 nm, and the light shield portion 406 has awidth of 20 nm. The second slit opening 404 has a width of 20 nm, theinterval between adjacent two of a plurality of second slit openings 404is 40 nm, and the crossed area 408 has a size of 20 nm×20 nm. In thiscase, the latent image pattern 405 formed in the photoresist is made upof a plurality of latent-dot-image formed areas 407 each having a sizeof about 30 nm×30 nm, which are arranged in a hexagonal lattice arraywith a pitch of 85 nm in the oblique direction and a pitch of 60 nm inthe horizontal direction as viewed in FIG. 4.

[0126] (Fourth Embodiment)

[0127]FIG. 5 shows a fourth embodiment of the opening pattern for thenear-field photomask of the present invention.

[0128] Referring to FIG. 5, areas 508 where light shield portions 506and second slit openings 504 in a near-field photomask 501 cross eachother are arrayed in a random pattern. By exposing a photoresist withthe use of the near-field photomask 501, a latent image pattern isformed in the photoresist as a dot pattern corresponding to the patternof the near-field photomask 501, as denoted by numeral 505.

[0129] The dimensions of the opening pattern formed in the near-fieldphotomask 501 are, by way of example, as follows. A row of first slitopening 503 has a width of 40 nm, and the light shield portion 506 has awidth of 20 to 100 nm. The second slit opening 504 has a width of 20 nm,and the crossed area 508 has a size of 20 nm×20 nm. In this case, thelatent image pattern 505 formed in the photoresist is made up of aplurality of latent-dot-image formed areas 507 each having a size ofabout 30 nm×30 nm, which are arranged in a pattern corresponding to thepattern of the near-field photomask 501.

[0130] A description is now made of examples of a device manufacturedwith the near-field exposure apparatus employing the near-fieldphotomask of the present invention. The present invention is suitablyapplied to any type of device so long as the device has a pattern ofdots each having a size of not larger than 100 nm. However, devicesdescribed below are particularly suitable for implementing the presentinvention.

[0131] (Fifth Embodiment)

[0132] As a first example, the present invention is suitable for formingquantum dots. One practical device is a super-lattice device in whichquantum dots each having a size of not larger than 100 nm areperiodically arrayed as shown in FIGS. 11A and 11B.

[0133] The quantum dot super-lattice device shown in FIGS. 11A and 11Bis of a structur that an active layer 1103 formed by burying GaAsquantum dots 1101 in i-type AlGaAs 1102 is sandwiched between an n-typeAlAs clad layer 1104 and a p-type AlAs clad layer 1105, and an n-sideelectrode 1106 and a p-side electrode 1107 are disposed on both sides ofthe sandwich-like members. Thus, this embodiment shows a light emittingdevice having luminous efficiency increased with that structure.

[0134] A method of manufacturing the quantum dot super-lattice device isas follows. The p-side electrode and the p-type AlAs clad layer areformed on a substrate, and i-type AlGaAs is grown at a predeterminedthickness on the p-type AlAs clad layer. Then, a GaAs layer of apredetermined thickness is grown at a predetermined thickness on thei-type AlGaAs. Subsequently, the GaAs layer is patterned into apredetermined two-dimensional dot array through successive processes ofphotoresist coating, exposure, development and etching by using theexposure apparatus and method of the present invention. GaAs quantumdots are thereby formed. A photomask used herein has a triangularlattice pattern as shown in FIG. 3. Further, i-type AlGaAs clad layer isgrown at a predetermined thickness on the patterned GaAs layer.Thereafter, the n-type AlAs clad layer and the n-side electrode areformed thereon.

[0135] (Sixth Embodiment)

[0136] As a second example, the present invention is suitable formanufacturing an optical device having a sub-wavelength structure (SWS).In the sub-wavelength structure, it is required to manufacturemicro-structures each having a size of not larger than 100 nm over awide region. One practical example of the optical device is anantireflection structured device shown in FIGS. 12A and 12B.

[0137] The antireflection structured device shown in FIGS. 12A and 12Bis constituted as a two-dimensional array of quartz-made conicalstructures 1202 formed on a quartz substrate 1201. By setting the pitchof the quartz-made conical structures 1202 to be smaller than thewavelength of a light incident upon the substrate, the antireflectionstructured device exhibits the function of not reflecting the incidentlight. Also, similarly to the first example, the antireflectionstructured device may be modified to have the function of partlyreflecting (scattering) the incident light, as required, by introducingdefects in a part of the quartz-made conical structures 1202.

[0138] A method of manufacturing the antireflection structured device isas follows. A photoresist is formed on the quartz substrate andpatterned into a predetermined two-dimensional dot array throughsuccessive processes of negative-type photoresist coating, exposure anddevelopment by using the exposure apparatus and method of the presentinvention. Then, the quartz substrate is etched by using, as a mask, thethus-obtained photoresist pattern in the form of two-dimensional dotarray. As a result, the conical structures each having a pointed apexand gradually spreading toward the foot are formed as shown in FIGS. 12Aand 12B.

[0139] (Seventh Embodiment)

[0140] As a third example, the present invention is suitable formanufacturing a sensor utilizing localized plasmon. For generating thelocalized plasmon, it is required to manufacture metallicmicro-structures each having a size of not larger than 100 nm over awide region. One practical example of such a sensor is a biosensor,shown in FIGS. 13A and 13B, in which a light is illuminated to metallicnano-particles, and an electric field is enhanced with localized plasmongenerated around each of the metallic micro-structures, therebyimproving sensitivity of the sensor.

[0141] The biosensor shown in FIGS. 13A and 13B is constituted as atwo-dimensional array comprising Au nano-particles 1302 formed on aglass substrate 1301 and sensor materials 1303 disposed around the Aunano-particles 1302. By illuminating a light for detection to thebiosensor, a sensor signal is obtained from a spectrum of the lighthaving passed through the two-dimensional array.

[0142] A method of manufacturing the biosensor is as follows. An Au thinfilm is formed on the glass substrate and patterned into a predeterminedtwo-dimensional dot array through successive processes of photoresistcoating, exposure, development and etching by using the exposureapparatus and method of the present invention. An Au nano-particle arrayis thereby obtained. Then, a sensor material is prepared by introducinga function group, which is capable of coupling with Au, into a materialmolecule, and is disposed so as to cover the surroundings of each Aunano-particle for coupling with Au.

[0143] Comparing with the structures of a similar device manufacturedusing the conventional self-assembly method, the structures manufacturedusing the present invention have higher regularity in period and size,whereby the device performance is improved. Further, local defects canfreely be introduced to the structures manufactured as per design sothat the device has a specific function.

[0144] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A near-field photomask made up of a light shieldfilm and openings formed in said light shield film, said photomask beingused to expose an exposure target with a near-field light generatedthrough the openings, wherein the openings formed in said light shieldfilm comprise two or more parallel rows of first slit openings eachhaving a width smaller than 100 nm, and a second slit opening having awidth smaller than 100 nm and extended perpendicularly to said rows offirst slit openings while interlinking at least two of said rows offirst slit openings.
 2. A near-field photomask according to claim 1,wherein an exposed area of said exposure target is given by an area ofsaid second slit opening which does not overlap with said first slitopening.
 3. A near-field photomask according to claim 1, wherein thewidth of said second slit opening is equal to a width of said lightshield film positioned between adjacent two of said first slit openings.4. A near-field photomask according to claim 3, wherein the exposed areaof said exposure target has a square dot pattern.
 5. A near-fieldphotomask according to claim 1, wherein a plurality second slit openingsare arranged at a predetermined interval.
 6. A near-field exposureapparatus comprising: a near-field photomask according to claim 1; lightilluminating means for illuminating a polarized light, which has anelectric field component parallel to said rows of first slit openings,to said near-field photomask; and means for positioning said near-fieldphotomask close to said exposure target up to a distance within anear-field region.
 7. A dot pattern forming method including a step offorming a dot pattern by using a near-field exposure apparatus accordingto claim
 6. 8. A dot pattern forming method according to claim 7,wherein a dot of said dot pattern is a quantum dot.
 9. A dot patternforming method according to claim 7, wherein a dot of said dot patternis a sub-wavelength structure.
 10. A dot pattern forming methodaccording to claim 7, wherein a dot of said dot pattern is a localizedplasmon generating structure.
 11. A dot pattern forming method accordingto claim 7, wherein said dot pattern has a plurality of dots formed inan array.
 12. A device manufactured by a dot pattern forming methodaccording to claim 7.